The present invention relates to a thin, lightweight, high energy density non-aqueous electrolyte secondary battery with simplified packaging, and an effective method for producing the same.
Recently, an increasing number of electronic appliances such as AV equipment and personal computers are becoming cordless and portable. With this development, high energy density non-aqueous electrolyte batteries containing a non-aqueous electrolyte are being widely adopted. Among them, lithium secondary battery is the most practically used non-aqueous electrolyte secondary battery.
For negative electrodes of these batteries, materials such as various graphites and amorphous carbons are employed. These are capable of absorbing and desorbing lithium as well as having a low electric potential close to that of lithium.
On the other hand, for the positive electrodes, materials such as lithium-containing transition metal compounds, for example, LiCoO2 and LiMn2O4, are employed. These are capable of absorbing and desorbing lithium as well as having a high electric potential with respect to lithium.
Electrode plates for these non-aqueous electrolyte batteries are produced, for example, in the following manner. First, to an electrode material, a conductive agent such as carbon fibers or carbon black, a reinforcing material such as a polymeric filler, a binder, a viscosity modifier and the like, and a slurry-like electrode mixture is prepared, using a solvent. This is coated onto a current collector or core material of any of a variety of forms such as a metal sheet, metal mesh, metal lath sheet or a punched metal. The resulting structure, if required, is rolled, dried and cut into a desired shape, thereby forming an electrode plate.
The non-aqueous electrolyte is prepared by dissolving a lithium salt such as LiPF6 or LiBF4 in a non-aqueous solvent. As the non-aqueous solvent, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, diethyl carbonate or the like is employed. Recently, a mixed solvent of a non-cyclic compound and a cyclic compound is frequently used.
With the recent trend for more compact equipment, there is a strong demand for lightweight batteries that can be accommodated in a space with limited area and volume. In addition, batteries having a sufficient energy density and a thickness of less than several millimeters are required in many cases.
For batteries containing a liquid non-aqueous electrolyte, it is necessary to prevent leakage of the non-aqueous electrolyte. It is also necessary to isolate the power generating elements, such as electrode plates and non-aqueous electrolyte, from the air containing moisture. For this reason, the power generating elements are housed in a container.
In the case of the initially adopted non-aqueous electrolyte batteries, an electrode plate group is formed, for example, by spirally winding or laminating electrode plates together with a separator. Subsequently, the electrode plate group is inserted into a cylindrical or square container, into which a non-aqueous electrolyte is injected. Then, the opening of the container is sealed with a sealing plate serving as an external terminal, thereby completing a battery. Such battery structure, however, is difficult to be designed thinner. Moreover, it is not very reliable in terms of electrolyte leakage.
Recently, polymer electrolytes in a gel-like form in which a liquid non-aqueous electrolyte is retained in a polymer matrix are being employed for batteries. Thin polymer batteries have also been developed, which are fabricated by laminating electrode plates with a separator layer comprising a polymer electrolyte interposed therebetween and covering the whole by a sheet-shaped outer jacket.
The separator layer comprising a polymer electrolyte is formed, for example, by causing a microporous membrane or non-woven fabric each containing a gel-forming agent to absorb a liquid non-aqueous electrolyte and sandwiching it between electrode plates. As the gel-forming agent, a polymer material that forms a gel electrolyte by absorbing a liquid non-aqueous electrolyte is employed.
A separator layer composed only of a polymer electrolyte can also be formed. An example of a method for this is mixing a gel-forming agent and a solvent to prepare a paste, laminating electrode plates with the paste interposed therebetween and drying the whole, followed by causing the gel-forming agent to absorb a liquid non-aqueous electrolyte. Additionally, another method is known, which involves mixing a gel-forming agent and a liquid non-aqueous electrolyte to prepare a paste and laminating electrode plates with the paste interposed therebetween.
The separator layer comprising a polymer electrolyte functions both as an electrolyte to transfer ions and as a separator to separate electrode plates. By housing into a sheet-shaped outer jacket, an electrode plate group formed by successively laminating a positive electrode, a separator layer comprising a polymer electrolyte and a negative electrode, it is possible to produce an extremely thin battery with high energy density.
Japanese Unexamined Patent Publication No. 2000-67850 discloses a technique of integrating electrode plates with a separator layer comprising a polymer electrolyte interposed therebetween.
For example, Japanese Unexamined Patent Publication No. 2000-12084 and Japanese Unexamined Patent Publication No. Hei 9-506208 disclose an electrode plate group having: a positive electrode plate comprising a current collector and an active material layer formed on one side thereof; a negative electrode plate comprising a current collector and an active material layer formed on one side thereof; and a separator layer comprising a polymer electrolyte interposed between the two active material layers.
Japanese Unexamined Patent Publication No. Hei 11-265699 discloses a structure in which electrode plates laminated with a separator layer comprising a polymer electrolyte interposed therebetween is housed in a bag-shaped outer jacket equipped with a safety venting mechanism.
For example, Japanese Unexamined Patent Publication Nos. 2000-156209 and 2000-223108 disclose an electrode plate group comprising: two electrode plates each comprising a current collector and an active material layer formed on one side thereof; a single electrode plate having an active material layer on each side thereof; and a separator layer interposed therebetween. This electrode plate group is covered by a sheet-shaped outer jacket.
In each of the above-described conventional thin batteries, the electrode plate group is housed in an outer jacket that is separately prepared. There is a limit to the possible reduction in thickness and improvement in energy density of batteries, as long as these are attempted based upon the idea of using a separately prepared outer jacket. There is also a limit to the possible simplification of packaging and of the manufacturing process of batteries.